1. Technical Field to Which the Invention Pertains
The present invention relates to an electroless gold plating solution for use in formation of a gold plate coating on an industrial electronic component such as a printed wiring board.
2. Prior Art
A printed board has a metal circuit pattern on and/or within the board, and a metal such as copper, which has a low electrical resistance, is used for the circuit. Furthermore, a nickel or nickel alloy barrier metal layer is provided for preventing oxidation and corrosion of the copper circuit and/or preventing migration of gold and, moreover, a gold coating is formed in order to suppress oxidation of nickel, maintain reliability of contacts, improve solderability, etc. When such a circuit is formed, in conventional methods, plating with nickel or a nickel alloy is carried out after forming the copper pattern, and there is then further carried out gold electroplating, autocatalytic gold plating after displacement gold plating, or thick displacement gold plating after the nickel.
With regard to a method for treating copper parts (terminals for connecting external parts or mounting components, through holes, etc.) exposed on the exterior of a printed board, copper wiring to which plating is applied is firstly subjected to pretreatments such as degreasing and etching, then to a palladium catalyst treatment, and to electroless nickel plating to form a barrier metal layer. With regard to a metal used as the barrier metal layer, palladium, platinum, silver, cobalt, or an alloy thereof can be used, as well as nickel or a nickel alloy. In order to prevent diffusion of nickel due to a thermal treatment, a technique of forming a palladium layer on a nickel layer has also been reported (K. Hasegawa et al., Proceeding of the 1997 IEMT/IMC, 230 (1997)).
The nickel, palladium, platinum, silver, cobalt, or an alloy thereof becomes a substrate metal for subsequent gold plating.
After the substrate metal layer is formed, it is covered with a gold coating, thus completing the circuit.
In general, since the gold coating is used for preventing corrosion of the circuit and/or is used as a contact, a coating having a high porosity is undesirable, and a surface having few gaps is required.
With regard to the main gold plating processes that have been carried out so far, there are 1) a method involving a two-stage process in which, after thin gold plating such as displacement gold plating or gold flashing is carried out, autocatalytic electroless gold plating is carried out to increase the coating thickness, 2) a method involving a one-stage process in which a thick coating is formed by thick displacement gold plating, and 3) a method in which a coating is formed by gold electroplating.
The displacement gold plating in 1) involves gold deposition by a galvanic displacement reaction between a substrate metal on the surface to be plated and gold ions and/or gold ion complexes, and the autocatalytic gold plating involves gold deposition by a reducing agent using gold as a catalyst. Examples of 1) are described in JP, A, 2001-185836, etc., and in this case since the gold coating formation process includes two steps, problems such as an increase in cost due simply to an increase in the number of steps occur easily.
In the case of the thick displacement gold plating in 2), it can be carried out in one step, but because of the characteristics of the displacement reaction, an oxide film is easily formed on the substrate metal surface and the porosity tends to become high.
In the case of the gold electroplating in 3), in general, a uniform coating cannot be formed on minute parts, making it difficult to obtain electrical continuity for a complicated circuit, and this method therefore cannot be put into practical use, which is a drawback.
In particular, in the case of the displacement gold plating, since nickel easily diffuses onto the gold plated surface, problems such as a deterioration of bonding performance easily occur (Hyomen Jissou Gijutsu (Surface Mounting Technology, Vol. 5 (11), 52 (1995)).
Furthermore, there is a method involving substrate catalyzed (surface catalyzed) gold plating by a reducing agent using a substrate metal as a catalyst (C. D. Iacovangelo 30 et al., U.S. Pat. No. 4,863,766), but since the substrate metal surface is coated with gold, once gold is coated, the thickness of the gold coating cannot be increased any more, and moreover the method involves a cyanide. The cyanide causes the problem of an increase in the cost of waste water treatment in addition to problems of storage and management and safety problems when carrying out various treatments. Because of this, there has been a desire for the development of an electroless gold plating solution containing no cyanide.
Furthermore, there is a gold plating solution with which gold plating can be carried out directly on gold and nickel using two types of reducing agents in combination, that is, a hydrazine type and a boron base type that have catalytic activity toward gold, nickel, palladium, etc. (J. Electrochem. Soc., Vol. 138, No. 4 (1991) 976-982; U.S. Pat. No. 4,979,988). This is gold plating by the so-called autocatalytic action and substrate metal catalyzed action; the use of such a gold plating solution cannot give a coating having a specific coating thickness, which can be obtained by displacement reaction gold plating, and the coating obtained cannot be said to have good adhesion. Furthermore, this gold plating solution contains a cyanide.
It is therefore an object of the present invention to provide an electroless gold plating solution that can solve the problems of each of the conventional electroless plating solutions and can form a uniform gold coating having good adhesion and low porosity in one step.